Patent GB-865765-A (GB)
Title: Improvements in or relating to apparatus for electrostatically depositing liquid coating material
IPC: B05B5/10
Filed: 1957-03-20
Abstract: <PICT:0865765/III/1> <PICT:0865765/III/2> <PICT:0865765/III/3> Apparatus for electrostatically painting an earthed article, capable of use where the gap between its charging electrode and the article or earth is liable to fortuitous variation, comprises a charging electrode, means connecting the article in an electric circuit from a unidirectional high potential source to the electrode, means for atomizing paint and introducing atomized paint into the space between the electrode and the article, the electrical circuit being so responsive to changes in the length of the gap as to reduce the voltage of the electrode sufficiently as the gap decreases, and the electrode and any other parts that are at high potential in use and are manually or otherwise externally accessible having an effective capacity that is sufficiently low to ensure that any discharge of electricity therefrom does not cause objectionable shock or fire hazard.  The circuit may be made so responsive by the inclusion therein of a high impedance, e.g. a resistance of 1500 megohms. Effective capacity is a measure of the ability of a discharge from the system to shock persons or ignite inflammable vapours and may be reduced below true capacitance by the use of a high distributed resistance between different parts of the capacitor.  As shown in Fig. 2, the gun 14 has a body with an outer housing 31 of polyethylene.  At one end is a rotating bell-like head 16 of nylon, polyethylene or similar insulating material, having its inner surface coated with a layer 16c of a material having chemical and mechanical resistance to the paint, and a high electrical resistance, e.g. an alkyd resin coating material containing finely divided carbon particles.  The head is rotated by an electric motor (not shown) through a hollow flexible shaft 24 which passes through the flexible conduit 23 and into the body of the gun.  The end of this shaft is connected, both mechanically and electrically with a hollow ceramic tube 29a with an outer casing 29b of high resistance material in a rigid carrier member 29c of synthetic resin.  The head 16 is mounted on the outer end of this member by rubber O-rings.  The conduit of the tube opens into the head at 16a, and the resistance layer 29b is connected with the layer 16c by a coating on the tip of tube 29a. Paint is supplied through the conduit in the hollow shaft 24 and tube 29a by means of a pump, not shown.  The resistance layer 29b, and the paint inside the tube 29a form two resistances in parallel which together provide the necessary high resistance. The flexible conduit 23 has an outer sheath 25 of polyethylene which extends into the body of the gun, and is held in place by the enlarged portion 33 moulded into it.  In another embodiment, shown in Fig. 5, a gun has three separate connections 41, 42 and 43 at its rear, all brought together into a single flexible conduit assembly covered with an earthed flexible metallic outer sheath 44.  A bell member 45 is mounted on a rotatable shaft 46 connected with a flexible drive shaft in the conduit 42.  The bell is of non-conducting material, e.g. nylon, with a surface layer 47 of high resistance material.  High voltage is introduced through the electrical conduit 41 and passes through two fixed resistors 48 and 49, e.g. of 500 megohms each, to a small metal leaf spring 50 in rubbing contact wth the resistive coating 47. Paint is supplied through the conduit 43 to the valve 54 operated by the trigger 51.  A passageway 57 leads from the valve 54 to the hollow shaft 46, permitting paint to reach the bell and spread outwards to the edge 45a where it is charged and atomized.  The valve 54 and handle 53 are both earthed.  In use, the electrical conduit 41 is connected with one terminal of a power pack, the other terminal of which is earthed.  The article to be coated is also earthed.  In a third embodiment, shown in Fig. 7, four small electrode wires 71a-d are arranged in a vertical plane with their ends one foot apart at the corners of a square.  Current is supplied from a high voltage source 74 through the control resistances 72a-d.  Paint from a conventional air-spray gun 75 is sprayed through the centre of the square and is charged thereby. Specifications 710,920, 865,764, 865,766, 865,767 and 865,768 also are referred to.